1. Field of the Invention
The present invention generally relates to the production of gypsum wallboard and, more specifically, the invention relates to a composition for accelerating the setting reaction of an aqueous calcined gypsum slurry, such as in the production of gypsum wallboard.
2. Brief Description of Related Technology
A common method of constructing walls and barriers includes the use of inorganic wallboard panels or sheets, such as gypsum wallboard, often referred to simply as "wallboard" or "drywall." Wallboard can be formulated for interior, exterior, and wet applications. The use of wallboard, as opposed to conventional boards made from wet plaster methods, is desirable because the installation of wallboard is ordinarily less costly and less cumbersome than installation of conventional plaster walls.
Generally, wallboard is produced by enclosing a core of an aqueous slurry of calcined gypsum and other materials between two large sheets of board cover paper. Various types of cover paper are known in the art. After the gypsum slurry has set (i.e., reacted with water present in the aqueous slurry) and dried, the formed sheet is cut into standard sizes. Methods for the production of gypsum wallboard generally are described, for example, by T. Michelsen, "Building Materials (Survey)," Encyclopedia of Chemical Technology, (1992 4th ed.), vol. 21, pp. 621-24, TP9.E685, the disclosure of which is hereby incorporated herein by reference.
Gypsum wallboard is manufactured utilizing commercial processes that are capable of operation under continuous, high-speed conditions. A conventional process for manufacturing the core composition of gypsum wallboard initially includes the premixing of dry ingredients in a high-speed mixing apparatus. The dry ingredients can include calcium sulfate hemihydrate (stucco), an accelerator, and a binder (e.g., starch). The dry ingredients are mixed together with a "wet" (aqueous) portion of the core composition in a pin mixer apparatus. The wet portion can include a first component, commonly referred to as a "paper pulp solution," that includes a mixture of water, paper pulp, and, optionally, one or more fluidity-increasing agents, and a set retarder. The paper pulp solution provides a major portion of the water that forms the gypsum slurry of the core composition. A second wet component can include a mixture of the aforementioned strengthening agent, foam, and other conventional additives, if desired. Together, the aforementioned dry and wet portions comprise an aqueous gypsum slurry that forms a wallboard core.
A major ingredient of the gypsum wallboard core is calcium sulfate hemihydrate, commonly referred to as "calcined gypsum," "stucco," or "plaster of Paris." Stucco has a number of desirable physical properties including, but not limited to, its fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH. Typically, stucco is prepared by drying, grinding, and calcining natural gypsum rock (i.e., calcium sulfate dihydrate). The drying step of stucco manufacture includes passing crude gypsum rock through a rotary kiln to remove any free moisture present in the rock from rain or snow, for example. The dried rock then is passed through a roller mill (or impact mill types of pulverizers), wherein the rock is ground or comminuted to a desired fineness. The degree of comminution is determined by the ultimate use. The dried, fine-ground gypsum can be referred to as "land plaster" regardless of its intended use. The land plaster is used as feed to calcination processes for conversion to stucco.
The calcination (or dehydration) step in the manufacture of stucco is performed by heating the land plaster, and generally can be described by the following chemical equation which shows that heating calcium sulfate dihydrate yields calcium sulfate hemihydrate (stucco) and water vapor: EQU CaSO.sub.4.2H.sub.2 O+heat.fwdarw.CaSO.sub.4.1/2H.sub.2 O+11/2H.sub.2 O.
This calcination process step is performed in a "calciner," of which there are several types known by those of skill in the art.
Uncalcined calcium sulfate (i.e., land plaster) is the "stable" form of gypsum. However, calcined gypsum, or stucco, has the desirable property of being chemically reactive with water, and will "set" rather quickly when the two are mixed together. This setting reaction is actually a reversal of the above-described chemical reaction performed during the calcination step. The setting reaction proceeds according to the following chemical equation which shows that the calcium sulfate hemihydrate is rehydrated to its dihydrate state: EQU CaSO.sub.4.1/2H.sub.2 O+11/2H.sub.2 O.fwdarw.CaSO.sub.4.2H.sub.2 O+heat
The actual time required to complete the setting reaction generally depends upon the type of calciner and the type of gypsum rock that are used to produce the gypsum, and can be controlled within certain limits by the use of additives such as retarders, set accelerators, and/or stabilizers, for example. Generally, the rehydration time period can be in a range of about two minutes to about eight hours depending on the amount and quality of retarders, set accelerators, and/or stabilizers present.
After the aqueous gypsum slurry is prepared, the slurry and other desired ingredients are continuously deposited to form a wallboard core between two continuously-supplied moving sheets of board cover paper. More particularly, the two cover sheets comprise a pre-folded face paper and a backing paper. As the slurry is deposited onto the face paper, the backing paper is brought down atop the deposited core slurry and bonded to the prefolded edges of the face paper. The whole assembly then is sized for thickness utilizing a roller bar or forming plate. The deposited core is then allowed to set between the two cover sheets, thereby forming a board. The continuously-produced board is cut into panels of a desired length and then passed through a drying kiln where excess water is removed to form a strong, dry, and rigid building material.
The cover sheets used in the process typically are multi-ply paper manufactured from re-pulped newspapers and/or other grades of recycled papers. The face paper has an unsized inner ply which contacts the core slurry such that gypsum crystals can grow up to (or into) the inner ply--this, along with the starch, is the principal form of bonding between the core slurry and the cover sheet. The middle plies are sized and an outer ply is more heavily sized and treated to control absorption of paints and sealers. The backing paper is also a similarly constructed multi-ply sheet. Both cover sheets must have sufficient permeability to allow for water vapor to pass therethrough during the downstream board drying step(s).
Standardized sheets (or panels) of wallboard typically are cut and trimmed to dimensions of about four feet (about 1.2 meters) wide and about 8 feet to about 16 feet (about 2.4 meters to about 4.9 meters) in length (ASTM-C36). Sheets typically are available in thicknesses varying in a range of about 1/4 inch to about one inch (about 0.6 centimeters to about 2.6 centimeters) in 1/8 inch increments. Standardized sheets of wallboard typically have a density in a range of about 1600 to about 1700 pounds (about 726 to about 772 kilograms) per thousand square feet (lbs/MSF) of one-half inch board.
The time at which the board may be cut, or in other words, the speed of the conveyor and the consequent rate of production of the gypsum board, is generally controlled by the setting time of the calcined gypsum slurry. Thus, conventional adjuvants to the calcined gypsum in the mixer generally include set time control agents, particularly accelerators. These and other additives such as pregenerated foam to control final density of the board, paper cover sheet bond promoting agents, fibrous reinforcements, consistency reducers and the like typically constitute less than 5%, and usually less than 2%, of the weight of the finished board core.
As noted above, a conventional adjuvant to the calcined gypsum slurry in the mixer is a set time control agent which can be used to decrease the setting time of the calcined gypsum slurry. Calcium sulfate hemihydrate mixed with an appropriate amount of water typically sets in about 25 minutes to a wallboard having a suitable hardness. In modem wallboard manufacturing plants, where a high production rate is necessary, a 25 minute setting period is too long and, therefore, it has been customary to reduce the setting time of the aqueous slurry to about five to about eight minutes by incorporating a gypsum set accelerator.
The most common form of accelerator used for setting a calcined gypsum slurry is calcium sulfate dihydrate. This material has been found to be an excellent accelerator, however, it is highly susceptible to becoming calcined, particularly in the presence of calcined gypsum which has been bagged while still warm or in the presence of moisture or heat generally. National Gypsum's Burkard et al. patents (U.S. Pat. Nos. 3,870,538; 3,920,465; and 4,019,920) each disclose a gypsum set accelerator comprising a mixture of calcium sulfate dihydrate and a water-soluble carbohydrate (e.g., starch). The presence of the water-soluble carbohydrate prevents the calcium sulfate dihydrate from undergoing an undesirable calcining reaction when exposed to the heat of a conventional comminuting ball mill. The accelerator is prepared by mixing the calcium sulfate dihydrate with the water-soluble carbohydrate and subjecting the mixture to the comminuting action of a compression mill, such as a rotary ball mill, a rod mill, or a tube mill, for a time sufficient to cause the two constituents to combine mechanically, and for a time sufficient to provide a final product having a particle surface area of about 10,000 square centimeters per gram (cm.sup.2 /g) as measured by Fisher Sub Sieve (FSS) techniques.
U.S. Pat. Nos. 3,947,285; 4,059,456; 4,132,565; and 4,298,394 can be considered cumulative to the Burkard et al. patents, discussed above, as they describe gypsum set accelerators comprising calcium sulfate dihydrate and water-soluble surfactants that are prepared by the comminuting action of a rotary ball mill to achieve a desired particle surface area. U.S. Pat. No. 3,797,758, teaches that a disintegration step is necessary to disintegrate agglomerates which form when a .alpha.-calcium sulfate hemihydrate (as opposed to a calcium sulfate dihydrate) is ground by the action of a vibrating ball mill. This disintegration step imparts undesired heat to the product.
U.S. Pat. Nos. 2,078,199 and 3,813,312 disclose that when calcium sulfate dihydrate mixed with a preservative, such as sugar or soluble dextrin, and then milled in a rotary ball mill dehydration of the accelerator is avoided even when subjected to various atmospheric conditions on storage. Additionally, U.S. Pat. No. 3,870,538 discloses that starch may be finely ground together with calcium sulfate dihydrate, which can serve as a preservative of the accelerator. However these materials, particularly sugar, are undesirably expensive.
In each of the foregoing cases of preparing an accelerator utilizing gypsum land plaster (i.e., calcium sulfate dihydrate), the comminuting action of a rotary ball mill to grind the land plaster into a finely particulate material undesirably generates high temperature heat, which must be removed during intermediate cooling stages of the process. Hence, a continuous process for manufacturing a gypsum set accelerator is not practically attainable. Furthermore, both of the high process temperatures and the lengthy cooling periods undesirably cause the accelerator to lose potency. This loss in potency leads to the use of greater amounts of the accelerator in order to achieve desired setting times.
Reference herein to the terms "set time" or "setting time," generally means the time between the event when the water first comes into contact with stucco or a mixture of the stucco and accelerator to form a gypsum slurry, and when calcium sulfate dihydrate crystals have sufficiently formed and have interlocked such that the slurry is not susceptible to flow. Those of skill in the art, however, are familiar with the metes and bounds of the terms.
In view of the foregoing, it would be desirable to provide a gypsum set accelerator having a potency superior to conventional gypsum set accelerators. Furthermore it would be desirable to provide a substantially continuous method of making a gypsum set accelerator having a potency superior to conventional gypsum set accelerators. Still further, it would be desirable to provide a gypsum wallboard prepared using a gypsum set accelerator having a potency superior to conventional gypsum set accelerators.